1. Field of the Invention
This invention relates to a combined generator and starter motor for an automobile.
2. Background of the Relevant Art
Electrical generators and starter motors are separate parts on most automobiles with internal combustion engines. However, several goals in the design of automobiles would be furthered by combining the generator and the starter motor on an automobile.
One such design goal is weight reduction. A combined generator and starter motor can reduce a vehicle's weight by eliminating components which are redundant to the generator and the starter motor. Examples of such components are the external case which each device has and the rotors and coils which each device also has. A combined generator and starter motor would be able to eliminate some of the components which are redundant to the two devices, providing the potential for vehicle weight reduction.
Another goal in the design of vehicles is cost reduction. A combined generator and starter motor would potentially cost less than the sum of the two devices individually. This cost reduction would be achieved through the elimination of redundant parts, the same elimination which could help reduce vehicle weight. Also, administrative costs of vehicle manufacturers can be reduced because only one part (a combined generator and starter motor) instead of two (a generator and a starter motor) needs to be purchased, stored, made available as a service part, and so on.
Yet another goal in the design of vehicles is reduction of vehicle assembly complexity. Fewer parts which go into the assembly of a vehicle often translate into lower cost for assembly labor to assemble the vehicle. Also, the likelihood of assembly mistakes is reduced if there are fewer parts to assemble into the vehicle.
Still another goal in the design of vehicles is high reliability. Axiomatic in reliability theory is the principle that greater complexity tends to cause lower reliability. A combined generator and starter motor will likely have fewer total components than the two devices separately. The combination of the generator and the starter motor would therefore have the potential for providing improved overall reliability of the vehicle.
U.S. Pat. No. 5,126,582 discloses a combined starter-generator. This device employs a basic planetary gear set. The device contains a rotor, the hub of which has external teeth to form a sun gear. Pinion gears attached to a carrier surround the sun gears. Finally, a stator contains an inner toothed surface which acts as a ring gear.
Although the invention in U.S. Pat. No. 5,126,582 does provide a combined generator and starter motor, it has three less-than-optimal features. First, the gear set, being a simple planetary gear set, does not provide as much speed reduction (and torque multiplication) as is potentially needed in some engine cranking applications. Second, in generating mode, the pinion gears continually revolve around the sun gear. Although the pinion gears are not transmitting torque (torque is transmitted directly from the engine through a shaft to the rotor), the continual rotation of the pinion gears can be a source of failure. Finally, the invention contains two air gaps, one between the rotor and a field coil and the other between the rotor and an armature. Two air gaps can cause an unnecessary loss of magnetic flux inside a motor or a generator, resulting in reduced efficiency.
Therefore, a combined generator and starter motor which provides large torque multiplication in starter motor mode, which reduces gear rotation in generator mode, and which has a single air gap will provide advantages over the prior art.